1. Field of the Invention
The present invention relates to a method for manufacturing a bonded wafer by an ion implantation delamination method.
2. Description of the Related Art
A method of manufacturing a SOI wafer by delaminating ions-implanted wafers after bonding, i.e., an ion implantation delamination method (a technique also referred to as the Smart Cut method (registered trademark)), has attracted attention as a method of manufacturing silicon on insulator (SOI) wafers, particularly a method of manufacturing SOI wafers having a thin SOI layer that enable improvement in performance of advanced integrated circuits.
This ion implantation delamination method is a technique to form an SOI wafer (See Patent Literature 1) in the following manner: an oxide film is formed on at least one of two silicon wafers; gas ions such as hydrogen ions or rare gas ions are implanted from a front surface of one of the silicon wafers (a bond wafer) to form the ion implantation layer (also referred to as a micro bubble layer or an enclosed layer) in the interior of the wafer; the surface from which the ions are implanted is then brought into close contact with the other silicon wafer (a base wafer) through the oxide film; a heat treatment (a delamination heat treatment) is then performed to cleave one of the wafers (the bond wafer) along the micro bubble layer so that the bond wafer is separated into a thin film; and another heat treatment (a bonding heat treatment) is then performed to strengthen a bond between the wafers. At this point, the cleavage plane (the delamination surface) is a surface of an SOI layer, and an SOI wafer having a thin SOI with high uniformity is relatively easily obtained.
The surface of the delaminated SOI wafer, however, contains a damaged layer due to ion implantation, and has surface roughness larger than the mirror surface of the usual silicon wafer. Accordingly, it is necessary to remove such damaged layer and surface roughness in an ion implantation delamination method.
Conventionally, mirror polishing with extremely small polishing stock removal (a stock removal of about 100 nm), referred to as touch polishing, has been performed in order to remove the damaged layer and the like in the final step after the bonding heat treatment. However, when polishing including a mechanical processing element is performed with respect to the SOI layer, since the stock removal of the polishing is not uniform, there occurs a problem that film thickness uniformity of the SOI layer achieved by implantation of hydrogen ions etc. and delamination is deteriorated.
For the purpose of solving such a problem, a flattening process involving heat treatment at a high temperature has been performed to improve the surface roughness instead of the touch polishing.
For example, Patent Literature 2 proposes to add a heat treatment (a rapid heating/rapid cooling heat treatment (an RTA treatment)) in a reducing atmosphere containing hydrogen without polishing a surface of an SOI layer after a delamination heat treatment (or a bonding heat treatment).
Herein, it is necessary to perform wet cleaning which is well-known and is called RCA cleaning prior to the heat treatment for the surface of the delaminated SOI layer (the delaminated layer) in order to avoid contamination such as particles or impurities as described in paragraph [0065] of Patent Literature 2.
Further, in paragraph [0050] of Patent Literature 3, the following subject matter is described: when manufacturing a SOI wafer having an oxide film on a terrace part (the peripheral part of a wafer where base wafer is exposed) by an ion implantation delamination method, silicon flakes will adhere onto an oxide film of a terrace part at the time that a bond wafer is delaminated and this causes particle contamination, etc. through the subsequent epitaxial growth. In order to avoid that, wet cleaning such as SC1 cleaning or HF cleaning are performed as a cleaning step to remove silicon flakes on the terrace part prior to an epitaxial growth.
Incidentally, wet cleaning described in Patent Literatures 2 and 3 commonly uses a cleaning method with applying ultrasonic to cleaning fluid to remove particle contaminants from an object to be cleaned by the vibrating action (ultrasonic cleaning) (see Patent Literatures 4 and 5).